Viscosity index improver, lubricant oil additive, and lubricant oil composition

ABSTRACT

A viscosity index improver of the present invention contains a star polymer obtained by a reaction of divinylbenzene with alkyl methacrylate containing stearyl methacrylate, the star polymer having a core part and an arm part, wherein the core part is derived from the divinylbenzene and the arm part is a polymer chain of the alkyl methacrylate. A lubricant oil additive and a lubricant oil composition of the present invention contain the above viscosity index improver.

TECHNICAL FIELD

The present invention relates to a viscosity index improver, a lubricantoil additive, and a lubricant oil composition. Specifically, the presentinvention relates to a viscosity index improver, a lubricant oiladditive, and a lubricant oil composition which are useful in the fieldof a lubricant oil for a fuel-efficient internal combustion engine orthe like.

BACKGROUND ART

In recent years, improvement in a fuel efficiency characteristic isrequired for a lubricant oil for an internal combustion engine,regardless of gasoline and a diesel. The required value also becomesstricter year after year, and cannot be utterly achieved in aconventional lubricant oil technique. Development of viscosity indeximprovers, which have the largest influence on the fuel efficiencycharacteristic, has been conducted in such a situation. Linearpolymethacrylate (also referred to as polymethacrylate, polymethacrylicacid alkyl ester, or PMA) and star polymethacrylate or the like havebeen developed.

For example, viscosity index improvers for a GTL base oil containing acopolymer containing (meth)acrylic acid alkyl ester having an alkylgroup having 1 carbon atom and/or 2 carbon atoms, (meth)acrylic acidalkyl ester having an alkyl group having 12 and/or 13 carbon atoms,(meth)acrylic acid alkyl ester having an alkyl group having 14 and/or 15carbon atoms, and (meth)acrylic acid alkyl ester having one or morealkyl groups selected from alkyl groups having 16-24 carbon atoms asindispensable structural monomers are disclosed in Patent Literature 1described below.

(a) A star polymer containing (i) a core portion containing a polyvalent(meth)acrylic monomer, an oligomer or polymer thereof, or a polyvalentdivinyl non-acrylic monomer, an oligomer or polymer thereof; and (ii) atleast two arms of polymerized (meth)acrylic acid alkyl ester, and (b) acomposition containing an oil having a lubricating viscosity, whereinthe core portion contains a specific functional group are disclosed inPatent Literature 2 described below. Specifically, a C12-C15/C8copolymer and a C12-C15/C1 copolymer or the like are shown as examplesof the core portion.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Unexamined Patent Application

Publication No. 2008-88215

-   Patent Literature 2: Japanese Unexamined Patent Application    Publication No. 2008-518052

SUMMARY OF INVENTION Technical Problem

However, there is still room for improvement in the viscosity indeximprover disclosed in Patent Literatures 1 and 2 as one capable of beingput to practical use. Although a viscosity-temperature characteristic isexcellent and an initial fuel efficiency characteristic is good, forexample, in the case of the linear polymethacrylate disclosed in PatentLiterature 1, shear stability is poor, which causes a problem that thefuel efficiency characteristic during long-term use is reduced. Althoughthe shear stability is excellent in the case of the starpolymethacrylate disclosed in Patent Literature 2, an original fuelefficiency characteristic improving effect is not sufficient.

Furthermore, recently, a long drain property has been also required fromthe viewpoint of the reduction in wastes in addition to the fuelefficiency characteristic. Because the lubricant oil is usually oxidizedand deteriorated by the use thereof, which makes it difficult tomaintain initial performance, it is questioned that the initial fuelefficiency characteristic is impaired with long-term use. Although thiscause relates to the stability of the viscosity index improver to beused, not simple stability but cutting of a polymer caused by receivingshear is questioned. Although it is required that a shear viscosity isreduced in a practical use region while the viscosity grade of SAE ismaintained for fuel efficiency, one satisfying the requirement is notyet found.

The present invention has been accomplished in light of thesecircumstances, and its object is to provide a viscosity index improverhaving an excellent viscosity-temperature characteristic, shearstability, and oxidation stability and capable of achieving a fuelefficiency characteristic and a long drain property, and a lubricant oiladditive and a lubricant oil composition using the viscosity indeximprover.

Solution to Problem

In order to solve the above problems, the present invention provides aviscosity index improver according to the following items [1] to [6], alubricant oil additive according to the following item [7], and alubricant oil composition according to the following items [8] and [9].

[1] A viscosity index improver containing a star polymer obtained by areaction of divinylbenzene with alkyl methacrylate containing stearylmethacrylate (also referred to as n-octadecyl methacrylate, stearylmethacrylate, or methacrylic acid n-octadecyl), the star polymer havinga core part and an arm part, wherein the core part is derived from thedivinylbenzene and the arm part is a polymer chain of the alkylmethacrylate.[2] The viscosity index improver according to the item [1], wherein aproportion of stearyl methacrylate in the alkyl methacrylate is 40 to100% by mass based on the total amount of the alkyl methacrylate.[3] The viscosity index improver according to the item [1] or [2],wherein a proportion of methyl methacrylate in the alkyl methacrylate is0 to 50% by mass based on the total amount of the alkyl methacrylate.[4] The viscosity index improver according to any one of the items [1]to [3], wherein a proportion of alkyl methacrylate having an alkyl grouphaving 10 to 16 carbon atoms in the alkyl methacrylate is 0 to 40% bymass based on the total amount of the alkyl methacrylate.[5] The viscosity index improver according to any one of the items [1]to [4], wherein the arm part has a number average molecular weight of10,000 to 60,000.[6] The viscosity index improver according to any one of the items [1]to [5], wherein the star polymer has a number average molecular weightof 50,000 to 1,000,000.[7] A lubricant oil additive containing the viscosity index improveraccording to any one of the items [1] to [6].[8] A lubricant oil composition containing a lubricating base oil andthe viscosity index improver according to any one of the items [1] to[6].[9] The lubricant oil composition according to the item [8], wherein thelubricant oil composition has a HTHS viscosity of 1.4 mPa·s or greaterat 150° C. and a HTHS viscosity of less than 5.4 mPa·s at 100° C.

Advantageous Effects of Invention

As described above, the present invention provides a viscosity indeximprover having an excellent viscosity-temperature characteristic, shearstability, and oxidation stability and capable of achieving a fuelefficiency characteristic and a long drain property, and a lubricant oiladditive and a lubricant oil composition using the viscosity indeximprover.

Furthermore, according to the present invention, the above effect can beobtained, regardless of whether a fuel used for an internal combustionengine is any of gasoline or diesel fuel.

DESCRIPTION OF EMBODIMENTS Example 1

Hereinafter, preferred embodiments of the present invention will bedescribed in detail.

First Embodiment Viscosity Index Improver

A viscosity index improver according to a first embodiment of thepresent invention contains a star polymer obtained by a reaction ofdivinylbenzene with alkyl methacrylate containing stearyl methacrylate,the star polymer having a core part and an arm part, wherein the corepart is derived from the divinylbenzene and the arm part is a polymerchain of the alkyl methacrylate.

The arm part of the star polymer is a polymer chain of the alkylmethacrylate containing stearyl methacrylate based on a vinyl group ofthe divinylbenzene. A proportion of stearyl methacrylate in the alkylmethacrylate contained in the arm part is preferably 40 to 100% by massbased on the total amount of the alkyl methacrylate, more preferably 45to 100% by mass, still more preferably 50 to 100% by mass, andparticularly preferably 55 to 100% by mass. The proportion of stearylmethacrylate in the above range can achieve a satisfactory balance withhigher levels of a viscosity-temperature characteristic, shearstability, and oxidation stability.

From the viewpoint of a fuel efficiency characteristic, the alkylmethacrylate contained in the arm part preferably contains methylmethacrylate, and a proportion of methyl methacrylate in this case ispreferably equal to or greater than 20% by mass based on the totalamount of the alkyl methacrylate. On the other hand, from the viewpointof achieving the satisfactory balance with higher levels of solubility,the viscosity-temperature characteristic, the shear stability, and theoxidation stability, the proportion of methyl methacrylate is preferablyequal to or less than 50% by mass, more preferably equal to or less than45% by mass, and still more preferably equal to or less than 43% bymass.

When the alkyl methacrylate contained in the arm part contains methylmethacrylate, the total of the proportions of stearyl methacrylate andmethyl methacrylate is preferably 60 to 100% by mass based on the totalamount of the alkyl methacrylate, more preferably 70 to 100% by mass,and still more preferably 80 to 100% by mass. Although the shearstability is excellent when the total of the proportions is less thanthe above lower limit, an original fuel efficiency characteristicimproving effect is potentially insufficient.

The alkyl methacrylate contained in the arm part may further containalkyl methacrylate having an alkyl group having 10 to 16 carbon atoms.Examples of the alkyl methacrylate include undecyl methacrylate, dodecylmethacrylate, tridecyl methacrylate, heptadecyl methacrylate, andhexadecyl methacrylate. In this case, a proportion of an alkylmethacrylate having an alkyl group having 10 to 16 carbon atoms in thealkyl methacrylate is preferably 0 to 40% by mass based on the totalamount of the alkyl methacrylate. When the proportion of the alkylmethacrylate having the alkyl group having 10 to 16 carbon atoms isgreater than the above upper limit, the solubility of the viscosityindex improver to a base oil tends to be reduced.

The number average molecular weight of the arm of the star polymeraccording to the present embodiment is preferably 10,000 to 60,000, morepreferably 12,000 to 50,000, still more preferably 13,000 to 40,000, andmost preferably 15,000 to 30,000. When the number average molecularweight of the arm part is less than the above lower limit, the viscosityis excessively increased, which potentially reduces the fuel efficiencycharacteristic, or when the number average molecular weight is greaterthan the above upper limit, the shear stability is potentially impaired.

The number average molecular weight of the star polymer is preferably50,000 to 1,000,000, more preferably 100,000 to 800,000, still morepreferably 120,000 to 600,000, and most preferably 150,000 to 400,000.When the number average molecular weight of the star polymer is lessthan the above lower limit, the addition amount of the viscosity indeximprover to a lubricating base oil required to obtain a desired effectincreases, which potentially reduces the oxidation stability of alubricant oil, or when the number average molecular weight is greaterthan the above upper limit, the addition amount decreases, but the shearstability is potentially reduced.

In the star polymer according to the embodiment, the average value ofthe number of the arm parts contained in a molecule (average arm number)is preferably 4 to 30, more preferably 5 to 25, and still morepreferably 6 to 20.

Although a method for synthesizing the star polymer according to theembodiment is not limited, examples of the method include a method forpolymerizing polyalkyl methacrylate which is the arm part (the polymerchain of the alkyl methacrylate) utilizing a controlled radicalpolymerization process, and thereafter reacting the polyalkylmethacrylate with divinylbenzene.

The controlled radical polymerization process includes an atom transferradical polymerization (ATRP) process, a reversible additionfragmentation chain transfer (RAFT) process, or a nitrogenoxide-mediated polymerization process.

The discussion of the polymer mechanism of ATRP polymerization is shownin reaction scheme 11.1 on page 524, in reaction scheme 11.4 on page566, in reaction scheme 11.7 on page 571, in reaction scheme 11.8 onpage 572, and in reaction scheme 11.9 on page 575 of Matyjaszewski etal.

The discussion of the polymer mechanism of RAFT polymerization is shownin Section 12.4.4 on pages 664-665 of Matyjaszewski et al.

The detailed descriptions of nitrogen oxide-mediated polymerization(Chapter 10, pages 463-522), ATRP (Chapter 11, pages 523-628) and RAFT(Chapter 12, pages 629-690) are shown in “Handbook of RadicalPolymerization” (Krzysztof Matyjaszewski and Thomas P. Davis, copyright2002, published by John Wiley and Sons Inc. (hereinafter referred to as“Matyjaszewski et al”)).

The above synthesis can be carried out as a batch operation, ahalf-batch operation, a continuous process, a feeding process, or a bulkprocess. This synthesis can be performed in an emulsion, a solution, ora suspension.

In the above synthesis, the average molecular weight of the obtainedpolymethacrylate or star polymer can be adjusted by changing the amountsof an initiator and the divinylbenzene (DVB) to be used.

The reaction rate to the star polymer using the synthesized arm part isequal to or greater than 70% based on the amount of the polymer reactedwith the star polymer, preferably equal to or greater than 80%, and morepreferably equal to or greater than 85%. When the reaction rate is low,the arm part is left, which cannot increase the molecular weight.

Second Embodiment Lubricant Oil Additive

A lubricant oil additive according to a second embodiment of the presentinvention contains the viscosity index improver according to the abovefirst embodiment.

The lubricant oil additive according to the embodiment may consistentirely of the viscosity index improver according to the above firstembodiment, or may be a mixture of the viscosity index improver andother additives.

The examples of the other additives include additives such as viscosityindex improvers other than the viscosity index improver according to theabove first embodiment, antioxidants, anti-wear agents (orextreme-pressure agents), corrosion inhibitors, rust-preventive agents,viscosity index improvers, pour point depressants, demulsifiers, metalinactivating agents, antifoaming agents, and ashless friction modifiers.These additives can be used singly or in combination of two or moretypes thereof.

Examples of the viscosity index improvers other than the viscosity indeximprover according to the above first embodiment includepolymethacrylate-based, polyisobutene-based, ethylene-propylenecopolymer-based, and styrene-butadiene hydrogenerated copolymer-basedviscosity index improvers.

Examples of the antioxidants include phenol-based and amine-basedashless antioxidants, and zinc-based, copper-based, and molybdenum-basedmetal antioxidants.

Preferred examples of the phenol-based antioxidants include4,4′-methylenebis(2,6-di-tert-butylphenol),4,4′-bis(2,6-di-tert-butylphenol),4,4′-bis(2-methyl-6-tert-butylphenol),2,2′-methylenebis(4-ethyl-6-tert-butylphenol),2,2′-methylenebis(4-methyl-6-tert-butylphenol),4,4′-butylidenebis(3-methyl-6-tert-butylphenol),4,4′-isopropylidenebis(2,6-di-tert-butylphenol),2,2′-methylenebis(4-methyl-6-nonylphenol),2,2′-isobutylidenebis(4,6-dimethylphenol),2,2′-methylenebis(4-methyl-6-cyclohexylphenol),2,6-di-tert-butyl-4-methylphenol, 2,6-di-tert-butyl-4-ethylphenol,2,4-dimethyl-6-tert-butylphenol, 2,6-di-tert-α-dimethylamino-p-cresol,2,6-di-tert-butyl-4-(N,N-dimethylaminomethylphenol),4,4′-thiobis(2-methyl-6-tert-butylphenol),4,4′-thiobis(3-methyl-6-tert-butylphenol),2,2′-thiobis(4-methyl-6-tert-butylphenol),bis(3-methyl-4-hydroxy-5-tert-butylbenzyl)sulfide,bis(3,5-di-tert-butyl-4-hydroxybenzyl)sulfide,2,2′-thio-diethylenebis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate],tridecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate,pentaerythrityl-tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate],octyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, stearyl3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate,octyl-3-(3-methyl-5-di-tert-butyl-4-hydroxyphenyl)propionate. These maybe used in a mixture of two or more types thereof.

Examples of the amine-based antioxidants include known amine-basedantioxidants generally used as lubricant oils such as aromatic aminecompounds, alkyldiphenylamine, alkylnaphthylamine,phenyl-α-naphthylamine, and alkylphenyl-α-naphthylamine.

Examples of the corrosion inhibitors include benzotriazole-based,tolyltriazole-based, thiadiazole-based, or imidazole-based compounds.

Examples of the rust-preventive agents include petroleum sulfonates,alkylbenzene sulfonates, dinonylnaphthalene sulfonates, alkenylsuccinicacid esters, or polyhydric alcohol esters.

Examples of the metal inactivating agents include imidazolines,pyrimidine derivatives, alkylthiadiazoles, mercaptobenzothiazoles,benzotriazole or its derivatives, 1,3,4-thiadiazolepolysulfide,1,3,4-thiadiazolyl-2,5-bisdialkyl dithiocarbamate,2-(alkyldithio)benzimidazole, or β-(o-carboxybenzylthio)propionitrile.

Examples of the antifoaming agents include silicone oils,alkenylsuccinic acid derivatives, polyhydroxyaliphatic alcohol andlong-chain fatty acid esters, methyl salicylate and o-hydroxybenzylalcohols, which have kinematic viscosities of 1,000 to 100,000 mm²/s at25° C.

Optional compounds usually used as the ashless friction modifiers forthe lubricant oil can be used as the ashless friction modifiers, andexamples of the ashless friction modifiers include ashless frictionmodifiers such as amine compounds, fatty acid esters, fatty acid amides,fatty acids, aliphatic alcohols, and aliphatic ethers with at least oneof alkyl groups or alkenyl groups having 6-30 carbon atoms, particularlystraight-chain alkyl groups or straight-chain alkenyl groups having 6-30carbon atoms in the molecule. Nitrogen-containing compounds, theiracid-modified derivatives and the like described in Japanese UnexaminedPatent Application Publication No. 2009-286831, and various ashlessfriction modifiers exemplified in International Patent Publication No.WO2005/037967 can also be used.

When the lubricant oil additive according to the embodiment is a mixtureof the viscosity index improver according to the above first embodimentand the other additive, the mixing proportion thereof is notparticularly limited, and can be suitably selected according to theobjective use.

Third Embodiment Lubricant Oil Composition

A lubricant oil composition according to a third embodiment of thepresent invention contains a lubricating base oil and the viscosityindex improver according to the above first embodiment. The lubricantoil composition may further contain the other additives exemplified inthe description of the above second embodiment.

The lubricating base oil in the embodiment is not particularly limited,and lubricating base oils used for usual lubricant oils can be used.Specifically, mineral oil-based lubricating base oils, syntheticoil-based lubricating base oils, or a mixture prepared by mixing two ormore types of lubricating base oils selected from these oils at anoptional proportion, or the like can be used.

Specific examples of the mineral oil-based lubricating base oils includebase oils refined by subjecting a lubricant oil fraction obtained byvacuum distillation of an atmospheric residue obtained by atmosphericdistillation of crude oil to one or more treatments such as solventdeasphalting, solvent extraction, hydrocracking, solvent dewaxing, andhydrorefining, or wax isomerizing mineral oils, base oils produced by atechnique for isomerizing GTL wax (gas-to-liquid wax).

Specific examples of the synthetic oil-based lubricant oils includepolybutenes or their hydrides; poly-α-olefins such as 1-octene oligomerand 1-decene oligomer or their hydrides; diesters such asditridecylglutarate, di-2-ethylhexyladipate, diisodecyladipate,ditridecyladipate, and di-2-ethylhexylsebacate; polyol esters such astrimethylolpropanecaprylate, trimethylolpropanepelargonate,pentaerythritol-2-ethylhexanoate, and pentaerythritolpelargonate; andaromatic synthetic oils such as alkylnaphthalenes and alkylbenzenes ortheir mixtures.

The kinematic viscosity of the lubricating base oil at 100° C. ispreferably 2.5 to 10.0 mm²/s, more preferably 3.0 to 8.0 mm²/s, andstill more preferably 3.5 to 6.0 mm²/s. The viscosity index of thelubricating base oil is preferably 90 to 165, more preferably 100 to155, and still more preferably 120 to 150.

Furthermore, in order to exhibit the effect of the additive, thesaturated component of the base oil obtained by the chromatographyanalysis is preferably equal to or greater than 80%, more preferablyequal to or greater than 85%, still more preferably equal to or greaterthan 90%, and most preferably equal to or greater than 95%.

The content of the viscosity index improver according to the firstembodiment is preferably 0.1 to 20.0% by mass based on the total amountof the lubricant oil composition, more preferably 0.5 to 15.0% by mass,and still more preferably 1.0 to 10.0% by mass. When the content is lessthan the above lower limit, a sufficient additive effect is notpotentially obtained. When the content is greater than the above upperlimit, the shear stability is reduced, which potentially impairs thedurability of the fuel efficiency.

The kinematic viscosity of the lubricant oil composition according tothe embodiment at 100° C. is preferably 3.0 to 16.3 mm²/s, morepreferably 3.5 to 12.5 mm²/s, and still more preferably 4.0 to 9.3mm²/s. When the kinematic viscosity at 100° C. is less than the abovelower limit, insufficient lubricity potentially results, or when thekinematic viscosity is greater than the above upper limit, a necessarylow-temperature viscosity and sufficient fuel efficiency performance arenot potentially obtained.

The viscosity index of the lubricant oil composition according to theembodiment is preferably 150 to 250, more preferably 160 to 240, andstill more preferably 170 to 230. When the viscosity index is less thanthe above lower limit, it is potentially difficult to improve the fuelefficiency characteristic while maintaining a HTHS viscosity, and it isalso potentially difficult to lower the low-temperature viscosity. Whenthe viscosity index is greater than the above upper limit, alow-temperature flow property is impaired, and further problemspotentially occur due to solubility of the additives or lack ofcompatibility with the sealant material.

The HTHS viscosity of the lubricant oil composition according to theembodiment at 150° C. is equal to or greater than 1.4 mPa·s, preferablyequal to or greater than 1.7 mPa·s, more preferably equal to or greaterthan 2.0 mPa·s, still more preferably equal to or greater than 2.3mPa·s, and most preferably equal to or greater than 2.6 mPa·s. The HTHSviscosity of the lubricant oil composition at 100° C. is preferably lessthan 5.4 mPa·s, and more preferably equal to or less than 5.2 mPa·s. TheHTHS viscosity at 150° C. or 100° C. herein designates ahigh-temperature high-shear viscosity at 150° C. or 100° C. according toASTM D4683. When the HTHS viscosity at 150° C. or 100° C. is greaterthan the above upper limit, the necessary low-temperature viscosity andsufficient fuel efficiency performance are not potentially obtained, orwhen the HTHS viscosity is less than the above lower limit,evaporativity is high, and insufficient lubricity potentially results.

The viscosity index improver according to the first embodiment, thelubricant oil additive according to the second embodiment, and thelubricant oil composition according to the third embodiment are notparticularly limited in their uses, and can be used in broad fields suchas a lubricant oil for an internal combustion engine and a drivingsystem lubricant oil.

Because the viscosity index improver according to the first embodiment,the lubricant oil additive according to the second embodiment, and thelubricant oil composition according to the third embodiment particularlyhave the excellent viscosity-temperature characteristic, shearstability, and oxidation stability, and can achieve the fuel efficiencycharacteristic and the long drain property, the viscosity indeximprover, the lubricant oil additive, and the lubricant oil compositionare useful in the field of the lubricant oil for the internal combustionengine. The fuel of the internal combustion engine in this case does nothave restrictions on gasoline or diesel fuel.

EXAMPLES

Hereinafter, based on Examples and Comparative Examples, the presentinvention will be more specifically described, but the present inventionwill not be limited to Examples below.

Example 1

A mixture of methyl methacrylate and stearyl methacrylate (mixingproportion: methyl methacrylate/stearyl methacrylate=60/40 (massratio)), an AIBN initiator (1 equivalent), cumyl dithiobenzoate (2equivalents), and an oil were mixed in a container (reaction flask)provided with a nitrogen inlet, a medium-speed machine stirrer, a thermocouple, and a water-cooled chiller under conditions of room temperatureand a nitrogen flow rate of 28.3 L/hr. At this time, they were stirredfor 20 minutes under a N₂ blanket in order to ensure the mixture. Then,the nitrogen flow rate was reduced to 14.2 L/hr, and the mixture washeated to 90° C. for 4 hours. Next, divinylbenzene was added into thecontainer, and the mixture was stirred at 90° C. for 12 hours to obtainan objective star polymer (hereinafter, sometimes referred to as PMA1).

For the obtained star polymer, the number average molecular weights ofthe arm part and the star polymer were measured by Gel PermeationChromatography (GPC, in terms of polystyrene). The average arm number ofthe star polymer and the inversion rate to the star polymer wereobtained based on the amount of a polymer added to the star polymer. Theobtained results are shown in Table 1.

A method for measuring a molecular weight was conducted as follows. Theobtained star polymer was dissolved in a THF solvent (tetrahydrofuran)so that the concentration was set to 1% by mass, and the measurement wasconducted at a flow rate of 0.5 ml/min and a column temperature of 40°C. using THF as a mobile phase and RI (refractive index) as a detector.A retention time of 10000 to 500000 was measured using polystyrene as acommercially available standard reagent for an analytical curve, and wasused to determine a molecular weight distribution.

Examples 2 and 3, Comparative Examples 1 to 3

In Examples 2 and 3 and Comparative Examples 1 to 3, star polymers weresynthesized in the same manner as in Example 1 except that alkylmethacrylates having compositions shown in Table 1 were used in place ofthe mixture of methyl methacrylate and stearyl methacrylate in Example 1(hereinafter, the obtained star polymers are possibly referred to asPMAs 2 to 6).

The number average molecular weight of the arm part, the number averagemolecular weight of the star polymer, the average arm number of the starpolymer, and the inversion rate to the star polymer in each of theobtained star polymers are shown in Table 1.

TABLE 1 Comparative Comparative Comparative Example 1 Example 2 Example3 Example 1 Example 2 Example 3 Star polymer PMA1 PMA2 PMA3 PMA4 PMA5PMA6 Composition of alkyl methacrylate (% by mass) Methyl methacrylate30 40 45 30 30 30 Lauryl methacrylate — — — 70 — — Palmityl methacrylate— — — — 70 — Stearyl methacrylate 70 60 55 — — — Icosyl methacrylate — —— — — 70 Number average molecular 19,500 18,700 17,500 18,300 18,80018,300 weight of arm part Polydispersity 1.15 1.14 1.15 1.13 1.14 1.15Number average molecular 180,000 182,000 168,000 186,000 192,000 178,000weight of star polymer Average arm number 9 10 10 10 10 10 Arm inversionrate (% by 87 89 88 91 86 82 mass)

Examples 4 to 6, Comparative Examples 4 to 6

In Examples 4 to 6 and Comparative Examples 4 to 6, a package additive(8% by mass: an ashless dispersant at 3% by mass, a metal-based cleaningagent at 3% by mass, ZDTP at 1% by mass, MoDTC at 0.4% by mass, and anamine-based antioxidant 0.6% by mass) was blended into group III baseoils (SK YUBASE-4) as lubricating base oils, and any one of PMAs 1 to 6was further added so as to satisfy 0W-20 as a standard of an engine oilto obtain lubricant oil compositions. The compositions and variousaspects of each of the lubricant oil compositions are shown in Table 2.“Y. S.” in Table 2 means yield stress.

TABLE 2 Comparative Comparative Comparative Example 4 Example 5 Example6 Example 4 Example 5 Example 6 Composition of a lubricant oilcomposition (% by mass) Lubricating base oil 88.7 88.8 88.4 88.7 88.687.9 Package additive 8.0 8.0 8.0 8.0 8.0 8.0 PMA1 3.3 — — — — — PMA2 —3.2 — — — — PMA3 — — 3.6 — — — PMA4 — — — 3.3 — — PMA5 — — — — 3.4 —PMA6 — — — — — 4.1 Appearance Transparent Transparent Cloudy CloudyTransparent Insoluble Kinematic viscosity at 37.3 34.2 33.9 38.8 37.939.1 40° C. (mm²/s) Kinematic viscosity at 8.05 7.45 7.3 7.7 7.65 7.99100° C. (mm²/s) Viscosity index 197 191 189 173 176 181 HTHS viscosityat 2.6 2.6 2.6 2.6 2.6 2.6 150° C. (mPa · s) HTHS viscosity at 5.3 5.25.1 5.8 5.7 5.8 100° C. (mPa · s) MRV viscosity (−40° C.) Existence orY.S. Y.S. Y.S. Y.S. Y.S. Y.S. existence nonexistence of Y.S.nonexistence nonexistence nonexistence nonexistence nonexistenceViscosity, mPa · s 14,000 12,800 10,300 >60,000 28,000 (inability tomeasure)

[Supersonic Shear Stability Test]

The shear stability of the lubricant oil composition of Example 4 wasevaluated by a SONIC method. A viscosity ratio before and after a test(a value obtained by dividing a kinematic viscosity after the test by akinematic viscosity before the test) is shown in Table 3.

Comparative Example 7

1,500 parts by mass of toluene were charged in a reaction containerprovided with a stirrer, a heating device, a chiller, a thermometer, anitrogen blow-in tube, and a drop tank; the reaction container wascontrolled to a temperature of 85° C.; and the reaction container wassubjected to nitrogen substitution for 60 minutes. Next, a mixture of1700 parts by mass of methyl methacrylate, 1020 parts by mass ofmethacrylic acid n-dodecyl ester, 255 parts by mass of methacrylic acid2-methyundecyl ester, 1020 parts by mass of methacrylic acid n-tridecylester, 255 parts by mass of methacrylic acid 2-methyldodecyl ester, 1224parts by mass of methacrylic acid n-tetradecyl ester, 306 parts by massof methacrylic acid 2-dodecyl ester, 1224 parts by mass of methacrylicacid n-pentadecyl ester, 306 parts by mass of methacrylic acid2-methyltetradecyl ester, and 830 parts by mass of methacrylic acidn-hexadecyl ester (a total of 8,500 parts by mass), and 4 parts by massof 2,2′-azobis(2,4-dimethylvaleronitrile) and 17 parts by mass of2,2′-azobis(2-methylbutyronitrile) as a polymerization initiator werecharged in the drop tank, and the solution of the drop tank was droppedat a constant speed for 2 hours at 85° C. under a sealed condition toperform a polymerization reaction. Continuously, ageing was performed at85° C. for 2 hours, and toluene was distilled away at a decompressiondegree of 20 mmHg at 120° C. to obtain linear polymethacrylate(hereinafter, possibly referred to as VM1).

Next, lubricant oil compositions shown in Table 3 were prepared usingthe obtained VMA1 as a viscosity index improver, and the same supersonicshear stability test as that in the above description was conducted. Theobtained results are shown in Table 3.

Comparative Example 8

A star polymer (hereinafter, possibly referred to as VM2) wassynthesized in the same manner as in Example 1 except that a mixture ofC₁₂₋₁₅ methacrylate (70% by weight) and 2-ethylhexyl methacrylate (30%by weight) was used in place of the mixture of methyl methacrylate andstearyl methacrylate in Example 1, and Trigonox (trademark)-21 (“T-21”)initiator (1 equivalent) was used in place of the AIBN initiator (1equivalent).

Next, a lubricant oil composition shown in Table 3 was prepared usingthe obtained VM2 as a viscosity index improver, and the same supersonicshear stability test as that in the above description was conducted. Theobtained results are shown in Table 3.

TABLE 3 Comparative Comparative Example 4 Example 7 Example 8 Type ofviscosity index PMA1 VM1 VM2 improver Composition of lubricant oilcomposition (%) Package additive 8.0 8.0 8.0 Viscosity index improver3.3 2.5 3.8 Lubricating base oil 88.7 89.5 88.2 Before supersonic shearKinematic viscosity at 40° C. 37.3 39.9 38.5 (mm²/s) Kinematic viscosityat 100° C. 8.05 8.48 7.98 (mm²/s) Viscosity index 197 197 186 Aftersupersonic shear Kinematic viscosity at 40° C. 37.2 38.8 38.0 (mm²/s)Kinematic viscosity at 100° C. 8.04 7.45 7.55 (mm²/s) Viscosity index196 162 171 Viscosity ratio (40° C.) 1.00 0.97 0.99 Viscosity ratio(100° C.) 1.00 0.88 0.95 Viscosity index ratio 1.00 0.82 0.92

1. A viscosity index improver comprising a star polymer obtainable by areaction of divinylbenzene with alkyl methacrylate containing stearylmethacrylate, the star polymer having a core part and an arm part,wherein the core part is derived from the divinylbenzene and the armpart is a polymer chain of the alkyl methacrylate.
 2. The viscosityindex improver according to claim 1, wherein a proportion of stearylmethacrylate in the alkyl methacrylate is 40 to 100% by mass based onthe total amount of the alkyl methacrylate.
 3. The viscosity indeximprover according to claim 1, wherein a proportion of methylmethacrylate in the alkyl methacrylate is 0 to 50% by mass based on thetotal amount of the alkyl methacrylate.
 4. The viscosity index improveraccording to claim 1, wherein a proportion of alkyl methacrylate havingan alkyl group having 10 to 16 carbon atoms in the alkyl methacrylate is0 to 40% by mass based on the total amount of the alkyl methacrylate. 5.The viscosity index improver according to claim 1, wherein the arm parthas a number average molecular weight of 10,000 to 60,000.
 6. Theviscosity index improver according to claim 1, wherein the star polymerhas a number average molecular weight of 50,000 to 1,000,000.
 7. Alubricant oil additive comprising the viscosity index improver accordingto claim
 1. 8. A lubricant oil composition comprising a lubricating baseoil and the viscosity index improver according to claim
 1. 9. Thelubricant oil composition according to claim 8, wherein the lubricantoil composition has a HTHS viscosity of 1.4 mPa·s or greater at 150° C.and a HTHS viscosity of less than 5.4 mPa·s at 100° C.